a methylated guanine nucleotide added to the 5' end of eukaryotic mRNA. The cap is necessary to initiate translation of mRNA

A band

The band of the sarcomere that extends the full length of the thick filament. The A band includes regions of thick and thin filament overlap, as well as a region of thick filament only. A bands alternate with I bands to give skeletal and cardiac muscle a striated apperance. The A band does not shorten during muscle contraction.

Absolute refractory period

A period of time following an action potential during which no additional action potential can be evoked regardless of the level of stimulation. (usually because Na+ channel closed whle K+ efflux)

Accessory glands

The three glands in the male reproductive system that reproduce semen: the seminal vesicles, the prostate, and the

Accessory organs

(1) In the GI tract, organs that play a role in digestion but not directly part of the alimentary canal. These include the liver, the gallbladder, the pancreas, adn the salivary glands.

Acetylcholine (Ach)

The neurotransmitter used throughout the parasympathetic nervous system as well as the neuromuscular junction.

Acetylcholinesterase (AChE)

The enzyme that breaks down acetylcholine in the synaptic cleft.

Acetyl-CoA

The first substrate in teh Krebs cycle, produced primarily from the oxidation of pyruvate by the pyruvate dehydrogenase complex, however acetyl-CoA is also produced during fatty acid oxidation and protein catabolism.

Acid hydrolases

Enzymes that degrade various macromolecules and that require an acidic pH to function properly. Acid hydrolases are found within the lysosomes of cells.

Acinar cells

Cells that make up exocrine galnds, adn that secrete their products into ducts. For example, in the pancreas, acinar cells secrete digestive enzyme; in the salivary glands, acinar cells secrete saliva.

Acrosome

A region at the head of a sperm cell that contains digestive enzyems which, when released during the acrosome reaction, can facilitate penetration of the corona radiata of the egg, and subsequently, fertilization

Actin

A contractile protein. In skeletal and cardiac muscle, actin polymerizes (along with other proteins) to form the thin filaments. Actin is involved in many contractile activities, such as cyotkinesis, pseudopod formation, and muscle contraction.

Action potential

A localized change in a neruon's or musce cell's membrane potential that can propogate itself away from its point of origin. Action potentials are an all-or-none process mediated by the opening of voltage-gated Na+ and K+ channels when the membrane is brought to the threshold potential; opening of the Na+ channels causes a characteristic depolarization, while opening of the K+ channels repolarizes the membrane.

Activation energy (Ea)

The amount of energy required to produce the transition state of a chemical reaction. If the activation energy for a reaction is very high, the reaction occurs very slowly. Enzymes (and other catalysts) increase reaction rates by reducing activation energy.

Active site

The 3D site of an enzyme where substrates (reactants) bind and a chemical reaction is facilitated.

Active transport

The movement of molecules through the plasma membrane against their concentration gradients. Active transport requires input of cellular energy, often in the form of ATP. An example is the Na+/K+ ATPase in the plasma membrane of all cells.

Adenine

One of the four aromatic bases found in DNA and RNA; also a component of ATP, NADH, and FADH2. Adenine is apurine; it pairs with thymine (in DNA) and with uracil (in RNA)

Adenohypophis

anterior pituitary gland

Adipocyte

fat cell

Adrenal medulla

The inner region of the adrenal gland. The adrenal medulla is part of the sympathetic nervous systme, and releases epinephrine (adrenaline) and norepinephrine into the blood when stimuated. These hormones augment and prolon the effects of sympathetic stimulation in the body.

Adrenergic tone

A constant input to the arteries that keeps them somewhat constricted to maintain a basal level of blood pressure.

Adrenocoricotropic hormone (ACTH)

A trop hormone produced by the anterior pituitary gand that targets the adrenal cortex, stimulating it to relase corisol and aldosterone.

Afferent arteriole

The small artery that carries blood toward the capillaries of the glomerulus.

Afferent neuron

A neuron that arries information (action potentials) to the central nervous system; a sensory neuron.

Albumin

A blood protein produced by the liver. Albumin helps to mantain blood osmotic pressure (oncotic pressure)

Aldosterone

The principal mineralocorticoid secreted by the adrenal cortex. This steroid hormone targets the kidney tubules and increases renal reabsorption of sodium, decreases reabsorption of K+, and causes secretion of ADH. The net effect is water reabsorption and blood pressure increase.

Alimentary canal

Also known as the gastrointestinal (GI) tract of the digestive tract, the alimentary canal is the long muscular "tube" that includes the mouth esophagus, somatch, small intesitne, and large intestine.

Allosteric regulation

The modifaction of enzyme activity through interactino of molecules with specific sites on the enzyme other than the active site (called allosteric sites)

Alveoli

(singular alveolus.) Tiny sacs, with walls only a single cell layer thick found at the end of the respiratory bronchiole tree. Alveoli are the site of gas exchange in the respiratory system.

Allele

A version of a gene. For example, the gene may be for eye color, and the allels include those for brown eyes, those for blu e eyes, those green eyes, etc. At most, dploid organsims can posses only two alleles for a given gene, one on each of the two homologous chromosomes.

Amino Acid

The monomer of a protein; amino acids hae an amio group on one end fo the molecule and a carboxylic acid group on the other, and of the of 2 different side chains.

Amino acid acceptor site

The 3' end of a tRNA molecule that binds an amino acid. The nucleotide sequence at this end is CCA

Aminoacyl tRNA

A tRNA with an amino acid attached. This is made by an animoacyl-tRNA synthetase specific to the amino acid being attache.d

Aminion

A sac filled with fluid (aminotic fluid) that surroudns and protects a developing embryo.

Amphipathic

The characteristics of amolecule that has both polar (hydrophilic) and non-polar hydrophobic) regions, e.g. phospholipids, bile, etc.

Amylase

An enzyme that digests starch into disaccharides. Amylase is secreted by salivary glands and by the pancreas.

Anabolism

The process of bulidng complex structures out of simpler precursors, e.g. synthesizing protiens from amino acids.

Analogous structures

Physical structures in two different organism that have funcitonal similarity due to their evoluntion in a common environment, but have different underlying structure. Analogous structures arise from convergent evolution.

Anal sphincter

The valve that controls the release of feces from the recturm. It has an internal part made of smooth muscle (thus involuntary) and an external part made of skeletal muscle (thus voluntary).

Anaphase

The third phase of mitosis. During anaphase, replicated chromosmes are split apart at their centromeres (the sister chromatids are separated from each other) and moved to opposite sides of the cell.

Anaphase I

The third phase of meiosis I. During anaphase I the rplicated homologous chromosomes are separated (the tetrad is split) and pulled to opposite sides of the cell.

Anaphase II

The third phase of meiosis II. During anaphase II the sister chromatids are finally spearated at their centromeres and puled to opposite sides of teh cell. Note that anaphase II is identical to mitotic anaphase, excep the number of chromosmes was reduced by half during meiosis I.

Androgens

Mal sex hormones. Testosteron is the primary androgen.

Angiotensin

A normal blood protein produced by the liver, angiotensin is converted to angiotensim I by renin (secreted by kidney when blood pressur falls). Angiotensin I si further onverted to angiotensim II by ACE (angiotensin converting enzyme). Angiotensin II is a powerful systemic vasocontrictor ans stimulator of aldosterone relase, both of which result in an increase in blood pressure.

Antagonist

Something that acts to oppose the action of something else. For example, muscles that move a join in oppoiste direction are said to be antagonists.

Anterioir pituitary gland

Also known as the adenohypophysis, the anterior pituitary is made of gland tissue and makes and secretes six different homrones: FSH, LH, ACTH, prolactin, TSH, and growth hormone. The anterior pituitary is controlled b yreleasing and inhibiting factors from the hypothalamus.

Antibody (Ab)

Also called immunoblobins, the antibodies are protiens secreted by B-cells upon activation that bind in a highly specific manner to foreign proteins (such as those found of the surface of pathogens or transplanted tissues). The foreign proteins are called antigens. Antibodies generally do not directly destroy antigens, rather they mark them for destruction through other methods, and can inativate antigens by clumping them together or by convering necessary active sites.

Anticodon

A sequence of three nucleotides (found int he anticodon loop of tRNA) that is complementary to a specific codon in mRNA. The codon to which the anticodon is complementary specifies the amino acid that is carried by that tRNA.

Antidiuretic hormone (ADH)

Water-soluble peptide hormone secreted by the posterior pituitary gland. It acts on DCT and collecting ducts via insertion of AQP2 and water reabsorption, concentrating the urine and increasing blood pressure.

Antigen (Ag)

A molecule (usually a protein) capable of initiating an immune repsonse (antibody production).